Magnetic field measuring device, magnetic field measurement method, and recording medium having recorded thereon magnetic field measurement program

ABSTRACT

A magnetic field measuring device that can measure a weaker magnetic field is provided. A magnetic field measuring device is provided, the magnetic field measuring device including: a sensor unit that has at least one magnetoresistive element; a reference voltage generating unit that outputs a reference voltage; a magnetic field generating unit that generates a magnetic field to be applied to the sensor unit; a feedback current generating unit that supplies, according to a difference between an output voltage of the sensor unit and the reference voltage, the magnetic field generating unit with a feedback current that generates a feedback magnetic field to diminish an input magnetic field to the sensor unit; a magnetic field measuring unit that outputs a measurement value corresponding to the feedback current; and an adjusting unit that uses the output voltage of the sensor unit to adjust the reference voltage.

The contents of the following Japanese patent application(s) areincorporated herein by reference:

2018-126201 filed in JP on Jul. 2, 2018

2019-093087 filed in JP on May 16, 2019

BACKGROUND 1. Technical Field

The present invention relates to a magnetic field measuring device, amagnetic field measurement method, and a recording medium havingrecorded thereon a magnetic field measurement program.

2. Related Art

There are known magnetic sensors in which one TMR element and threefixed resistors are used to form a bridge circuit, an electrical powerfor causing a current to flow through a magnetic field generating coilis generated based on an output voltage of the bridge circuit, and amagnetic field is applied to the TMR module by using the magnetic fieldgenerating coil (see Patent Literature 1, for example). In addition,there are known magnetic sensors in which one TMR element and threefixed resistors are used to form a bridge circuit, and a voltage to beapplied to the bridge circuit is controlled based on an output voltageof the bridge circuit (see Patent Literature 2, for example).

Patent Literature 1: Japanese Patent Application Publication No.2017-083173

Patent Literature 2: Japanese Patent Application Publication No.2017-096627

SUMMARY

In conventional magnetic sensors, the magnetic operating point of a TMRelement shifts to a magnetic saturation region where the magneticresolution lowers depending on the balance between individual resistancevalues of resistors constituting a bridge circuit. However, for examplein biomagnetic field measurement such as magnetocardiographicmeasurement, it is desired to realize a magnetic field measuring devicethat can measure a weaker magnetic field.

In order to overcome the drawbacks explained above, a first aspect ofthe present invention provides a magnetic field measuring device. Themagnetic field measuring device may include a sensor unit that has atleast one magnetoresistive element. The magnetic field measuring devicemay include a reference voltage generating unit that outputs a referencevoltage. The magnetic field measuring device may include a magneticfield generating unit that generates a magnetic field to be applied tothe sensor unit. The magnetic field measuring device may include afeedback current generating unit that supplies, according to adifference between an output voltage of the sensor unit and thereference voltage, the magnetic field generating unit with a feedbackcurrent that generates a feedback magnetic field to diminish an inputmagnetic field to the sensor unit. The magnetic field measuring devicemay include a magnetic field measuring unit that outputs a measurementvalue corresponding to the feedback current. The magnetic fieldmeasuring device may include an adjusting unit that uses the outputvoltage of the sensor unit to adjust the reference voltage.

In an adjustment phase, the adjusting unit may adjust the referencevoltage, and in a measurement phase, the magnetic field measuring unitmay output a measurement value corresponding to the feedback currentgenerated for a measurement-target magnetic field.

The reference voltage generating unit may have at least one variableresistor, and the adjusting unit may change a resistance value of thevariable resistor to adjust the reference voltage.

The adjusting unit may adjust the reference voltage based on thefeedback current.

Upon the sensor unit receiving an adjustment magnetic field, theadjusting unit may adjust the reference voltage such that themeasurement value falls within a predetermined range as a result of thereception of the adjustment magnetic field.

The adjusting unit may adjust the reference voltage so as to lower avariance of the feedback current.

The magnetic field measuring device may further include a switching unitthat switches whether to or not to supply the feedback current to themagnetic field generating unit, the adjusting unit uses the outputvoltage of the sensor unit generated while the feedback current is notbeing supplied to the magnetic field generating unit to adjust thereference voltage.

Upon the sensor unit receiving an adjustment magnetic field while thefeedback current is not being supplied to the magnetic field generatingunit, the adjusting unit may adjust the reference voltage such that thedifference between the output voltage of the sensor unit and thereference voltage falls within a predetermined range as a result of thereception of the adjustment magnetic field.

The magnetic field measuring device may further include an adjustmentcurrent generating unit that generates an adjustment current, whereinthe switching unit supplies the adjustment current to the magnetic fieldgenerating unit if the feedback current is not supplied to the magneticfield generating unit, and the adjusting unit uses the output voltage ofthe sensor unit generated while the adjustment current is not beingsupplied to the magnetic field generating unit to adjust the referencevoltage.

The adjusting unit may adjust the reference voltage based on acharacteristic of the difference between the reference voltage and theoutput voltage of the sensor unit generated corresponding to theadjustment current.

The magnetic field measuring unit may integrate measurement valuesobtained in a predetermined period, and output the integratedmeasurement values.

Before measurement of a measurement-target magnetic field performed bythe magnetic field measuring unit, the adjusting unit may make thereference voltage generating unit generate the reference voltage thatmakes the feedback current generating unit generate a reset magneticfield to magnetically saturate the magnetoresistive element.

Before measurement of a measurement-target magnetic field performed bythe magnetic field measuring unit, the adjusting unit may: change aresistance value of a variable resistor provided in the referencevoltage generating unit to make the reference voltage generating unitgenerate a reset magnetic field generating voltage that generates thereset magnetic field; and adjust the reference voltage based on theresistance value of the variable resistor that generates the resetmagnetic field generating voltage.

The adjusting unit may adjust the reference voltage by using theresistance value of the variable resistor which is set to ½ to ¼ of arange of an upper reset magnetic field generating resistance value and alower magnetic field generating resistance value each of which generatesthe reset magnetic field generating voltage.

An output voltage range of the reference voltage generating unit may belarger than an output voltage range of the sensor unit.

The magnetic field measuring device may further include a reset currentgenerating unit that supplies, before measurement of ameasurement-target magnetic field performed by the magnetic fieldmeasuring unit, the magnetic field generating unit with a reset currentthat generates a reset magnetic field to magnetically saturate themagnetoresistive element.

The magnetic field measuring device may further include a high-passfilter that allows passage therethrough of a high-frequency component ofa measurement value output by the magnetic field measuring unit.

The feedback current generating unit may be formed by using two or moreoperational amplifiers.

The sensor unit may include a magnetic flux concentrating unit arrangedadjacent to the magnetoresistive element, and the feedback currentgenerating unit may be formed to surround the magnetoresistive elementand the magnetic flux concentrating unit.

The magnetoresistive element may include a magnetization free layer, anon-magnetic layer, and a magnetization fixed layer that are stacked ona substrate in this order, and, when seen from above, the area of themagnetization fixed layer may be smaller than the area of themagnetization free layer, and a magnetosensitive area may be determinedbased on the area of the magnetization fixed layer.

The sensor unit may have a first magnetoresistive element and a secondmagnetoresistive element that are connected in series and have oppositepolarity to each other, and a voltage across the first magnetoresistiveelement and the second magnetoresistive element may be output.

A second aspect of the present invention provides a magnetic fieldmeasurement method by which a magnetic field measuring device measures amagnetic field. The magnetic field measurement method may include:supplying, by the magnetic field measuring device and according to adifference between an output voltage of a sensor unit having at leastone magnetoresistive element and a reference voltage output by areference voltage generating unit, a magnetic field generating unit thatgenerates a magnetic field to be applied to the sensor unit with afeedback current that generates a feedback magnetic field to diminish aninput magnetic field to the sensor unit. The magnetic field measurementmethod may include outputting a measurement value corresponding to thefeedback current by the magnetic field measuring device. The magneticfield measurement method may include adjusting, by the magnetic fieldmeasuring device, the reference voltage based on the output voltage ofthe sensor unit.

A third aspect of the present invention provides a recording mediumhaving recorded thereon a magnetic field measurement program. Themagnetic field measurement program may be executed by a computer. Themagnetic field measurement program may make the computer function as afeedback current generating unit that supplies, according to adifference between an output voltage of a sensor unit having at leastone magnetoresistive element and a reference voltage output by areference voltage generating unit, a magnetic field generating unit thatgenerates a magnetic field to be applied to the sensor unit with afeedback current that generates a feedback magnetic field to diminish aninput magnetic field to the sensor unit. The magnetic field measurementprogram may make the computer function as a magnetic field measuringunit that outputs a measurement value corresponding to the feedbackcurrent. The magnetic field measurement program may make the computerfunction as an adjusting unit that uses the output voltage of the sensorunit to adjust the reference voltage.

The summary clause does not necessarily describe all necessary featuresof the embodiments of the present invention. The present invention mayalso be a sub-combination of the features described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the configuration of a magnetic field measuringdevice 10 according to the present embodiment.

FIG. 2 illustrates an example of the magnetic field measuring device 10according to the present embodiment in which a reference voltagegenerating unit 120 has at least one variable resistor.

FIG. 3 illustrates a first exemplary flow of magnetic operating pointadjustment performed by the magnetic field measuring device 10 accordingto the present embodiment.

FIG. 4 illustrates characteristics of voltage Vopen generatedcorresponding to an input magnetic field Bin before and after magneticoperating point adjustment performed by the magnetic field measuringdevice 10 according to the present embodiment based on the flowillustrated in FIG. 3.

FIG. 5 illustrates a second exemplary flow of magnetic operating pointadjustment performed by the magnetic field measuring device 10 accordingto the present embodiment.

FIG. 6 illustrates characteristics of voltage Vopen generatedcorresponding to an input magnetic field Bin before and after magneticoperating point adjustment performed by the magnetic field measuringdevice 10 according to the present embodiment based on the flowillustrated in FIG. 5.

FIG. 7 illustrates the configuration of the magnetic field measuringdevice 10 provided with a switching unit 710 according to a variant ofthe magnetic field measuring device 10 of the present embodiment.

FIG. 8 illustrates a third exemplary flow of magnetic operating pointadjustment performed by the magnetic field measuring device 10illustrated in FIG. 7.

FIG. 9 illustrates characteristics of voltage Vopen generatedcorresponding to an input magnetic field Bin before and after magneticoperating point adjustment performed by the magnetic field measuringdevice 10 illustrated in FIG. 7 based on the flow illustrated in FIG. 8.

FIG. 10 illustrates the configuration of the magnetic field measuringdevice 10 provided with the switching unit 710 and an adjustment currentgenerating unit 1010 according to a variant of the magnetic fieldmeasuring device 10 of the present embodiment.

FIG. 11 illustrates a fourth exemplary flow of magnetic operating pointadjustment performed by the magnetic field measuring device 10illustrated in FIG. 10.

FIG. 12 illustrates characteristics of voltage Vopen generatedcorresponding to an input magnetic field Bin before and after magneticoperating point adjustment performed by the magnetic field measuringdevice 10 illustrated in FIG. 10 based on the flow illustrated in FIG.11.

FIG. 13 illustrates characteristics of voltage Vopen generatedcorresponding to an adjustment current Iadjust used by an adjusting unit170 in the magnetic field measuring device 10 illustrated in FIG. 10 forcalculating voltage Vopen_adjust.

FIG. 14 illustrates dVopen/dIadjust characteristics used by theadjusting unit 170 in the magnetic field measuring device 10 illustratedin FIG. 10 for calculating voltage Vopen_adjust.

FIG. 15 illustrates a flow for the magnetic field measuring device 10according to the present embodiment to measure a magnetic field.

FIG. 16 illustrates the configuration of the magnetic field measuringdevice 10 provided with the switching unit 710 and a reset currentgenerating unit 1610 according to a variant of the magnetic fieldmeasuring device 10 of the present embodiment.

FIG. 17 illustrates how voltage Vclosed changes when the resistancevalue of the variable resistor 224 is changed in the magnetic fieldmeasuring device 10 according to the present embodiment.

FIG. 18 illustrates the configuration of the magnetic field measuringdevice 10 provided with a switch 1710 and a high-pass filter 1720according to a variant of the magnetic field measuring device 10 of thepresent embodiment.

FIG. 19 illustrates the configuration of the magnetic field measuringdevice 10 provided with a third operational amplifier 1910 according toa variant of the magnetic field measuring device 10 of the presentembodiment.

FIG. 20 illustrates a specific example of a sensor unit 110 according tothe present embodiment.

FIG. 21 illustrates a magnetic flux distribution observed when afeedback magnetic field is generated to the sensor unit 110 according tothe present specific example.

FIG. 22 illustrates an exemplary configuration of the sensor unit 110according to the present specific example.

FIG. 23 shows an example of a computer 2200 in which aspects of thepresent invention may be wholly or partly embodied.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, (some) embodiment(s) of the present invention will bedescribed. The embodiment(s) do(es) not limit the invention according tothe claims, and all the combinations of the features described in theembodiment(s) are not necessarily essential to means provided by aspectsof the invention.

FIG. 1 illustrates the configuration of a magnetic field measuringdevice 10 according to the present embodiment. The magnetic fieldmeasuring device 10 includes a sensor unit 110, a reference voltagegenerating unit 120, a feedback current generating unit 130, a magneticfield generating unit 140, and an operating unit 150.

The sensor unit 110 has at least one magnetoresistive element. Althoughfor example the present embodiment illustrates an example the sensorunit 110 has a first magnetoresistive element 112 and a secondmagnetoresistive element 114 which are connected in series between powersupply voltage Vcc and ground GND, and a voltage across the firstmagnetoresistive element 112 and the second magnetoresistive element 114is output, instead of this, in the sensor unit 110, for example, one ofthe first magnetoresistive element 112 and the second magnetoresistiveelement 114 may be constituted by a fixed resistor. There are variouspossible aspects in which the sensor unit 110 outputs a voltagecorresponding to a magnetic field input to at least one magnetoresistiveelement.

However, if the sensor unit 110 is configured to have the firstmagnetoresistive element 112 and the second magnetoresistive element 114that are connected in series and have opposite polarity to each other,and to output a voltage across the first magnetoresistive element 112and the second magnetoresistive element 114, this is more preferablesince an effect of reducing variations of characteristics such as offsetor sensitivity characteristics due to temperature can be attained. Here,having opposite polarity means that the resistance of a magnetoresistiveelement increases, and the resistance of the other magnetoresistiveelement decreases in response to magnetic fields input in the samedirection.

The first magnetoresistive element 112, and the second magnetoresistiveelement 114 may be, for example, tunnel magneto-resistance (TMR)elements, giant magneto-resistance (GMR) elements, or the like.

The reference voltage generating unit 120 outputs a reference voltage.The reference voltage generating unit 120 is configured to be able toadjust the reference voltage to be output. The reference voltagegenerating unit 120 supplies the adjusted reference voltage to thefeedback current generating unit 130.

The feedback current generating unit 130 supplies, according to thedifference between the output voltage of the sensor unit 110 and thereference voltage output by the reference voltage generating unit 120,the magnetic field generating unit 140 with a feedback current thatgenerates a feedback magnetic field to diminish an input magnetic fieldto the sensor unit 110. In the present embodiment, for example, thefeedback current generating unit 130 has a first operational amplifier132 that has two differential input terminals that are connected to theoutput voltage of the sensor unit 110 and the output of the referencevoltage generating unit 120 (i.e., the reference voltage), respectively.Then, the first operational amplifier 132 generates a feedback currentcorresponding to the difference between the output voltage of the sensorunit 110 and the reference voltage, and supplies the feedback current tothe magnetic field generating unit 140. Here, the difference between theoutput voltage of the sensor unit 110 and the reference voltage isdefined as Vopen.

The magnetic field generating unit 140 generates a feedback magneticfield to be applied to the sensor unit 110. In the present embodiment,for example, the magnetic field generating unit 140 has a coil 142. If afeedback current is supplied from the feedback current generating unit130, based on the supplied feedback current, the coil 142 generates afeedback magnetic field to be applied to the first magnetoresistiveelement 112 and second magnetoresistive element 114 provided in thesensor unit 110. Here, the sensor unit 110 (and the reference voltagegenerating unit 120) may be positioned to be enclosed by the coil 142.

The operating unit 150 has a current voltage conversion resistor 152, asecond operational amplifier 154, an AD converter 156, a magnetic fieldmeasuring unit 160, and an adjusting unit 170, and performs varioustypes of operations related to the magnetic field measuring device 10.

The current voltage conversion resistor 152 has one end connected to themagnetic field generating unit 140, and another end connected to a fixedvoltage 1. The current voltage conversion resistor 152 converts afeedback current into a voltage, and generates, across its both ends, avoltage based on the feedback current (feedback current x resistancevalue of the current voltage conversion resistor 152). Here, the voltagebased on the feedback current generated by the current voltageconversion resistor 152 is defined as Vclosed.

The second operational amplifier 154 has a differential input terminalconnected to both ends of the current voltage conversion resistor 152,and outputs a voltage VAMP corresponding to the voltage across both endsof the current voltage conversion resistor 152, that is, the voltageVclosed.

The AD converter 156 is connected to the second operational amplifier154, and converts, into a digital value VADC, the analog voltage valueVAMP corresponding to the voltage Vclosed output by the secondoperational amplifier 154.

In a measurement phase, the magnetic field measuring unit 160 outputs ameasurement value corresponding to the feedback current. In the presentembodiment, for example, the magnetic field measuring unit 160 isconnected to the AD converter 156, and outputs a measurement value basedon the digital value VADC that is obtained through conversion by the ADconverter 156 and corresponds to the voltage Vclosed.

In an adjustment phase, the adjusting unit 170 uses the output voltageof the sensor unit 110 to adjust the reference voltage output by thereference voltage generating unit 120. This is described below. Notethat although the explanation described above illustrated an example inwhich the magnetic field measuring unit 160 and the adjusting unit 170are configured as separate functional units, the magnetic fieldmeasuring unit 160 and the adjusting unit 170 may be configured as anintegrated functional unit.

By using the magnetic field measuring device 10 according to the presentembodiment, if a measurement-target magnetic field is input to thesensor unit 110, the feedback current generating unit 130 generates afeedback current corresponding to the difference between the referencevoltage and the output voltage of the sensor unit 110 generatedcorresponding to the measurement-target magnetic field (that is, thevoltage Vopen), and supplies the feedback current to the magnetic fieldgenerating unit 140. Then, according to the supplied feedback current,the magnetic field generating unit 140 generates a feedback magneticfield to cancel out the measurement-target magnetic field input to thesensor unit 110. Then, in a measurement phase, the magnetic fieldmeasuring unit 160 outputs a measurement value corresponding to thefeedback current generated for the measurement-target magnetic field,specifically, a digital value VADC corresponding to the voltage Vclosed.Here, this series of control is defined as closed-loop control. Notethat under the closed-loop control, control is performed such that thevalue of the voltage Vopen becomes 0, that is, a feedback magnetic fieldto cancel out an input magnetic field is generated.

FIG. 2 illustrates an example of the magnetic field measuring device 10according to the present embodiment in which the reference voltagegenerating unit 120 has at least one variable resistor. As illustratedin this figure, the reference voltage generating unit 120 has at leastone variable resistor. For example, the reference voltage generatingunit 120 may be configured to have a fixed resistor 222 and a variableresistor 224 connected in series between the power supply voltage Vccand the ground GND, and output a voltage across the fixed resistor 222and the variable resistor 224 as the reference voltage. In addition, asillustrated in this figure, the first magnetoresistive element 112, andsecond magnetoresistive element 114 provided in the sensor unit 110, andthe fixed resistor 222 and variable resistor 224 provided in thereference voltage generating unit 120 may constitute a bridge circuit210. Other than this, the reference voltage generating unit 120 mayhave, as the fixed resistor 222, a magnetoresistive element having thesame polarity as the second magnetoresistive element 114 (polarityopposite to the first magnetoresistive element 112), and have, as thevariable resistor 224, a configuration in which a variable resistor anda magnetoresistive element having the same polarity as the firstmagnetoresistive element 112 (polarity opposite to the secondmagnetoresistive element 114) are connected in series. If the referencevoltage generating unit 120 has a variable resistor, the adjusting unit170 changes the resistance value of the variable resistor to adjust thereference voltage output by the reference voltage generating unit 120(the reference voltage as a voltage obtained through resistivedivision).

FIG. 3 illustrates a first exemplary flow of magnetic operating pointadjustment performed by the magnetic field measuring device 10 accordingto the present embodiment. The magnetic operating point is defined asthe total of magnetic fields input to magnetoresistive elementsconstituting the magnetic field measuring device 10 according to thepresent embodiment. At Step 310, for example, a measurer sets an inputmagnetic field to be input to the sensor unit 110 to an adjustmentmagnetic field having a value predetermined for performing magneticoperating point adjustment. Here, the adjustment magnetic field may haveany value within a magnetic field range that the sensor unit 110 canperform magnetic detection. In the example explained below, there are noapplied adjustment magnetic fields. If there are no applied adjustmentmagnetic fields, for example, a measurer places the magnetic fieldmeasuring device 10 according to the present embodiment in a magneticshield room or a portable magnetic shield box to thereby shield themagnetic field measuring device 10 from environmental magnetic fieldssuch as the geomagnetic field such that there are no applied inputmagnetic fields input to the sensor unit 110.

Next, at Step 320, the adjusting unit 170 acquires the digital valueVADC that is based on the voltage Vclosed in the state where anadjustment magnetic field having a predetermined value is being input tothe sensor unit 110.

Then, at Step 330, the adjusting unit 170 adjusts the reference voltageoutput by the reference voltage generating unit 120 based on a feedbackcurrent. In this flow, upon the sensor unit 110 receiving an adjustmentmagnetic field, the adjusting unit 170 adjusts the reference voltageoutput by the reference voltage generating unit 120 such that thedigital value VADC that is based on a measurement value, for example,the voltage Vclosed, falls within a predetermined range of values as aresult of the reception of the adjustment magnetic field, and theadjusting unit 170 ends the process. For example, the adjusting unit 170adjusts the reference voltage output by the reference voltage generatingunit 120 such that, for example, the digital value VADC that is based onthe voltage Vclosed becomes equal to or smaller than a predeterminedthreshold so as to make the voltage Vclosed 0 if there are no appliedadjustment magnetic fields input to the sensor unit 110. Note that ifthere is an applied adjustment magnetic field, the adjusting unit 170adjusts the reference voltage output by the reference voltage generatingunit 120 such that the voltage Vclosed becomes a value corresponding tothe strength of the adjustment magnetic field. Note that in the closedloop, a voltage Vclosed and a voltage VAMP correspond to each otheruniquely, and may be treated as equivalent physical quantities.

FIG. 4 illustrates characteristics of voltage Vopen generatedcorresponding to an input magnetic field Bin before and after magneticoperating point adjustment performed by the magnetic field measuringdevice 10 according to the present embodiment based on the flowillustrated in FIG. 3. A curve 410 illustrates characteristics of thevoltage Vopen generated corresponding to the input magnetic field Bin tothe sensor unit 110 before magnetic operating point adjustment based onthe flow illustrated in FIG. 3. For example, if there are no appliedinput magnetic fields Bin to the sensor unit 110, the value of Vopenshould be 0 provided that the output voltage of the sensor unit 110 andthe reference voltage are ideally the same value. However, the outputvoltage of the sensor unit 110 and the reference voltage output by thereference voltage generating unit 120 do not necessarily become ideallythe same value due to fluctuations in element formation processes of thefirst magnetoresistive element 112 and second magnetoresistive element114 provided in the sensor unit 110, and the fixed resistor 222 andvariable resistor 224 provided in the reference voltage generating unit120, and the like, for example. As a result, even if there are noapplied input magnetic fields Bin, the value of the voltage Vopen doesnot become 0, and may assume a finite value (defined as “Vinitial”) asillustrated by a point 420, for example. The magnetic operating point atthis time is the point 420.

If the closed-loop control is performed in this state, the feedbackcurrent generating unit 130 generates a feedback currentIfeedback_initial corresponding to the voltage Vinitial, and suppliesthe feedback current Ifeedback_initial to the magnetic field generatingunit 140. Then, based on this feedback current Ifeedback_initial, themagnetic field generating unit 140 generates a feedback magnetic fieldBfeedback_initial so as to make the voltage Vopen 0. That is, due to thefeedback magnetic field Bfeedback_initial, the voltage Vopen becomes 0,and the magnetic operating point transitions from the point 420 to apoint 430. If the magnetic field measuring device 10 measures ameasurement-target magnetic field in this state, the firstmagnetoresistive element 112 and the second magnetoresistive element 114perform measurement of the magnetic field while the magnetic operatingpoint is at the point 430.

However, characteristics of the voltage Vopen generated corresponding tothe input magnetic field Bin have magnetic saturation regions asillustrated in FIG. 4, in accordance with the magnetic saturationcharacteristics of the first magnetoresistive element 112 and secondmagnetoresistive element 114 provided in the sensor unit 110. Then, thefirst magnetoresistive element 112 and the second magnetoresistiveelement 114, if operated in the magnetic saturation regions or nearbyregions, become unable to achieve a high magnetic sensitivity (the rateof change of the voltage Vopen in response to the magnetic field Bin),and to detect a weak measurement-target magnetic field. In view of this,in an adjustment phase before a measurement phase, the magnetic fieldmeasuring device 10 in the present embodiment adjusts the magneticoperating point of the first magnetoresistive element 112 and the secondmagnetoresistive element 114 to thereby make it possible for the firstmagnetoresistive element 112 and the second magnetoresistive element 114to operate at a point where they can achieve a relatively high magneticsensitivity, that is, a high magnetic resolution.

A curve 440 illustrates characteristics of the voltage Vopen generatedcorresponding to the input magnetic field Bin to the sensor unit 110after magnetic operating point adjustment based on the flow illustratedin FIG. 3. The magnetic field measuring device 10 according to thepresent embodiment performs magnetic operating point adjustment based onthe flow illustrated in FIG. 3 to thereby be able to cause a transitionof the operating point of the first magnetoresistive element 112 and thesecond magnetoresistive element 114 from the point 430 to a point 450.This point 450 is a point where the voltage Vclosed becomes 0, that is,the feedback current becomes 0 if there are no applied input magneticfields Bin, and, if the first magnetoresistive element 112 and thesecond magnetoresistive element 114 are operated at this point, thehighest magnetic sensitivity can be achieved.

In a conventional magnetic sensor that uses a bridge circuit constitutedby one TMR element and three fixed resistors, the magnetic operatingpoint of the TMR element is positioned in a magnetic saturation regionwhere the magnetic sensitivity is lowered due to fluctuations in elementformation processes of the TMR element and the fixed resistors, and thelike in some cases. In contrast to this, the magnetic field measuringdevice 10 of the present embodiment can cause a transition of themagnetic operating point of the first magnetoresistive element 112 andthe second magnetoresistive element 114 to a point where the magneticsensitivity is relatively high, and can detect a weakermeasurement-target magnetic field as a signal.

FIG. 5 illustrates a second exemplary flow of magnetic operating pointadjustment performed by the magnetic field measuring device 10 accordingto the present embodiment. In the magnetic operating point adjustment inthis flow, the input magnetic field Bin to be input to the sensor unit110 needs not be set to an adjustment magnetic field having apredetermined value, unlike the magnetic operating point adjustment inthe flow illustrated in FIG. 3. That is, for example, a measurer needsnot to place the magnetic field measuring device 10 in a shield room ora portable shield box such that there are no applied input magneticfields input to the sensor unit 110. At Step 510, the adjusting unit 170acquires the digital value VADC that is based on the voltage Vclosed.Note that the input magnetic field input to the sensor unit 110 at thistime point has not a predetermined known value, but an unknown value, asmentioned above.

Next, at Step 520, the adjusting unit 170 calculates the variance of thevoltage Vclosed acquired at Step 510. Here, a variance indicates themagnitude of fluctuations of values that the voltage Vclosed can assumein a predetermined period. For example, the adjusting unit 170 mayacquire values of the voltage Vclosed in a predetermined period,calculate their average value, square the difference between the valueof each Vclosed and the average value, and take the average of thethus-obtained values to thereby calculate the variance of the voltageVclosed.

Then, at Step 530, the adjusting unit 170 adjusts the reference voltageoutput by the reference voltage generating unit 120 so as to lower thevariance of the voltage Vclosed, and ends the process. For example, theadjusting unit 170 adjusts the reference voltage output by the referencevoltage generating unit 120 so as to minimize the variance of thevoltage Vclosed calculated at Step 520. Note that since the voltageVclosed is a voltage obtained through conversion of a feedback currentvia the current voltage conversion resistor 152, minimizing the varianceof the voltage Vclosed corresponds to minimizing the variance of thefeedback current.

FIG. 6 illustrates characteristics of voltage Vopen generatedcorresponding to an input magnetic field Bin before and after magneticoperating point adjustment performed by the magnetic field measuringdevice 10 according to the present embodiment based on the flowillustrated in FIG. 5. Since explanations similar to those related toFIG. 4 apply to portions given the same symbols as those illustrated inFIG. 4, those explanations are omitted. A difference from FIG. 4 is thatthe input magnetic field Bin input to the sensor unit 110 has not aknown value, but a finite unknown value (defined as “Bsignal”).

If the closed-loop control is performed in this state, the feedbackcurrent generating unit 130 generates the feedback current Ifeedback forcancelling out a magnetic field Bsignal in addition to the feedbackcurrent Ifeedback_initial corresponding to the voltage Vinitial, andsupplies them to the magnetic field generating unit 140. Then, based onthese feedback currents, the magnetic field generating unit 140generates the feedback magnetic field Bfeedback_initial so as to makethe voltage Vopen 0, and also generates the feedback magnetic fieldBfeedback so as to cancel out the magnetic field Bsignal. In this casealso, the magnetic operating point of the first magnetoresistive element112 and the second magnetoresistive element 114 is the point 430,similar to FIG. 4. Here, the adjusting unit 170 adjusts the referencevoltage output by the reference voltage generating unit 120 so as toadjust the magnetic operating point of the first magnetoresistiveelement 112 and the second magnetoresistive element 114 based on thefeedback currents, but the adjusting unit 170 cannot distinguish betweenthe feedback currents, Ifeedback_initial and Ifeedback.

In view of this, in the present embodiment, the adjusting unit 170adjusts the reference voltage based on the variance of the voltageVclosed. Typically, since as the magnetic operating point of amagnetoresistive element approaches a magnetic saturation point, themagnetic sensitivity lowers, it has characteristics that the ratio offluctuations of output to the magnetic sensitivity (uncertainty ofoutput) increases (that is, the signal noise ratio in magnetic detectionlowers) as the magnetic operating point approaches a magnetic saturationpoint. Then, since in the present embodiment, the feedback currentsgenerated by the feedback current generating unit 130 are based on theoutput voltage of the sensor unit 110 having the first magnetoresistiveelement 112 and the second magnetoresistive element 114, they reflectthe signal noise ratio in magnetic detection by the firstmagnetoresistive element 112 and the second magnetoresistive element114. For example, since as the magnetic operating point of the firstmagnetoresistive element 112 and the second magnetoresistive element 114approaches a magnetic saturation point, the signal noise ratio lowers,fluctuations of the feedback currents increase following the loweringsignal noise ratio. Accordingly, if the reference voltage is adjusted soas to reduce fluctuations of the feedback currents, it becomes possibleto cause a transition of the magnetic operating point of the firstmagnetoresistive element 112 and the second magnetoresistive element 114to a point farthest from a magnetic saturation point, that is, a pointat which they can have the highest magnetic sensitivity. Utilizing thisprinciple, the adjusting unit 170 adjusts the reference voltage outputby the reference voltage generating unit 120 so as to reduce thevariance of the voltage Vclosed reflecting the variance of the feedbackcurrents, and causes a transition of the magnetic operating point of thefirst magnetoresistive element 112 and the second magnetoresistiveelement 114 to a point where they can have a relatively high magneticsensitivity.

A curve 640 illustrates characteristics of the voltage Vopen generatedcorresponding to the input magnetic field Bin to the sensor unit 110after magnetic operating point adjustment based on the flow illustratedin FIG. 5. The magnetic field measuring device 10 according to thepresent embodiment performs magnetic operating point adjustment based onthe flow illustrated in FIG. 5 to thereby be able to cause a transitionof the magnetic operating point of the first magnetoresistive element112 and the second magnetoresistive element 114 from the point 430 to apoint 650. This point 650 is a point where the variance of the voltageVclosed becomes the smallest, that is, the variance of the feedbackcurrent becomes the smallest, and, if the first magnetoresistive element112 and the second magnetoresistive element 114 are operated at thispoint, the highest magnetic sensitivity can be achieved.

Although the explanation described above illustrated as an example atechnique of lowering the variance of the voltage Vclosed reflecting thevariance of the feedback current, this is not the sole example. Forexample, instead of lowering the variance of the voltage Vclosed, themagnetic field measuring device 10 may adjust the reference voltage soas to lower the peak-to-peak value of the signal amplitude of thevoltage Vclosed. Alternatively, for example, the magnetic fieldmeasuring device 10 may use a signal analyser (or a FFT analyzer, etc.)on measurement data of the voltage Vclosed along the time axis tomonitor the signal-strength frequency dependence (e.g., the frequencydependence of the electrical power density, etc.) of the voltageVclosed. Then, since this signal-strength frequency dependence of thevoltage Vclosed represents the signal-strength frequency dependence offluctuations of output of the magnetoresistive elements, the magneticfield measuring device 10 may adjust the reference voltage so as tolower the degree of the signal-strength frequency dependence of thefluctuations. Note that this technique of analyzing the frequencydependence can be performed by using not a signal analyser, but aprocessor such as a microcomputer.

FIG. 7 illustrates the configuration of the magnetic field measuringdevice 10 provided with a switching unit 710 according to a variant ofthe magnetic field measuring device 10 of the present embodiment. Themagnetic field measuring device 10 illustrated in this figure furtherincludes the switching unit 710 in addition to the configurations of themagnetic field measuring device 10 illustrated in FIG. 1. The switchingunit 710 is provided between the feedback current generating unit 130and the magnetic field generating unit 140, and can switch whether to ornot to supply a feedback current generated by the feedback currentgenerating unit 130 to the magnetic field generating unit 140. Inaddition, the switching unit 710 can supply output of the feedbackcurrent generating unit 130 to the AD converter 156 if a feedbackcurrent is not supplied to the magnetic field generating unit 140. Inthis case, the adjusting unit 170 uses the output voltage of the sensorunit 110 in the state where a feedback current is not being supplied tothe magnetic field generating unit 140 to adjust the reference voltageoutput by the reference voltage generating unit 120.

FIG. 8 illustrates a third exemplary flow of magnetic operating pointadjustment performed by the magnetic field measuring device 10illustrated in FIG. 7. Here, the state where the closed-loop control isnot being performed is defined as an open loop. At Step 810, theswitching unit 710 switches the state of control from the closed-loopcontrol to the state where a feedback current is not supplied to themagnetic field generating unit 140, that is, the open loop. In addition,the switching unit 710 supplies output of the feedback currentgenerating unit 130 to the AD converter 156.

At Step 820, similar to step 310 illustrated in FIG. 3, for example, ameasurer sets an input magnetic field to be input to the sensor unit 110to an adjustment magnetic field having a value predetermined forperforming magnetic operating point adjustment. Note that in this casealso, preferably there are no applied adjustment magnetic fields. In theexample explained below, there are no applied adjustment magneticfields.

Next, at Step 830, the adjusting unit 170 acquires the value of thevoltage Vopen in the state where an adjustment magnetic field having apredetermined value is being input to the sensor unit 110. For example,through digital conversion of output of the feedback current generatingunit 130 by the AD converter 156, the adjusting unit 170 acquires adigital value VADC that is based on the voltage Vopen. Note that in theopen loop, a voltage Vopen and a digital value VADC correspond to eachother uniquely, and may be treated as equivalent physical quantities.

Then, at Step 840, upon the sensor unit 110 receiving an adjustmentmagnetic field while the feedback currents are not being supplied to themagnetic field generating unit 140, the adjusting unit 170 adjusts thereference voltage output by the reference voltage generating unit 120such that the voltage Vopen which is the difference between the outputvoltage of the sensor unit 110 and the reference voltage output by thereference voltage generating unit 120 falls within a determined range asa result of the reception of the adjustment magnetic field, and ends theprocess. For example, the adjusting unit 170 adjusts the referencevoltage output by the reference voltage generating unit 120 such that,for example, the absolute value of the voltage Vopen becomes equal to orsmaller than a predetermined threshold so as to make the voltage Vopen 0if there are no applied adjustment magnetic fields input to the sensorunit 110.

FIG. 9 illustrates characteristics of voltage Vopen generatedcorresponding to an input magnetic field Bin before and after magneticoperating point adjustment performed by the magnetic field measuringdevice 10 illustrated in FIG. 7 based on the flow illustrated in FIG. 8.Since explanations similar to those related to FIG. 4 apply to portionsgiven the same symbols as those illustrated in FIG. 4, thoseexplanations are omitted. A difference from FIG. 4 is that the magneticfield measuring device 10 performs magnetic operating point adjustmentin the open loop.

Since, if the switching unit 710 switches the state of control from theclosed-loop control to the open loop, the feedback magnetic fieldBfeedback_initial is no longer generated, the magnetic operating pointof the first magnetoresistive element 112 and the secondmagnetoresistive element 114 transitions from the point 430 to the point420. In this state, for example, the adjusting unit 170 adjusts thereference voltage output by the reference voltage generating unit 120 soas to make the voltage Vopen 0 if there are no applied input magneticfields input to the sensor unit 110. A curve 940 illustratescharacteristics of the voltage Vopen generated corresponding to theinput magnetic field Bin to the sensor unit 110 after magnetic operatingpoint adjustment based on the flow illustrated in FIG. 8. The magneticfield measuring device 10 illustrated in FIG. 7 performs magneticoperating point adjustment based on the flow illustrated in FIG. 8 tothereby be able to cause a transition of the magnetic operating point ofthe first magnetoresistive element 112 and the second magnetoresistiveelement 114 from the point 420 to a point 950. This point 950 is a pointwhere the voltage Vopen becomes 0 when there are no applied inputmagnetic fields Bin, and if the magnetic field measuring device 10illustrated in FIG. 7 switches the state of control from the open loopto the closed loop in this state, and operates the firstmagnetoresistive element 112 and the second magnetoresistive element114, the highest magnetic sensitivity can be attained.

FIG. 10 illustrates the configuration of the magnetic field measuringdevice 10 provided with the switching unit 710 and an adjustment currentgenerating unit 1010 according to a variant of the magnetic fieldmeasuring device 10 of the present embodiment. The magnetic fieldmeasuring device 10 illustrated in this figure further includes theadjustment current generating unit 1010 in addition to theconfigurations of the magnetic field measuring device 10 illustrated inFIG. 7. The adjustment current generating unit 1010 generates anadjustment current Iadjust. In addition, the switching unit 710 of themagnetic field measuring device 10 in this figure can supply output ofthe feedback current generating unit 130 to the AD converter 156 if afeedback current is not supplied to the magnetic field generating unit140, and also make the magnetic field generating unit 140 connected tothe adjustment current generating unit 1010 to supply an adjustmentcurrent to the magnetic field generating unit 140. In this case, theadjusting unit 170 uses the output voltage of the sensor unit 110 in thestate where an adjustment current is being supplied to the magneticfield generating unit 140 to adjust the reference voltage output by thereference voltage generating unit 120.

FIG. 11 illustrates a fourth exemplary flow of magnetic operating pointadjustment performed by the magnetic field measuring device 10illustrated in FIG. 10. At Step 1110, similar to Step 810, the switchingunit 710 switches the state of control from the closed-loop control tothe open loop. In addition, the switching unit 710 supplies output ofthe feedback current generating unit 130 to the AD converter 156, andalso make the magnetic field generating unit 140 connected to theadjustment current generating unit 1010 to supply an adjustment currentto the magnetic field generating unit 140.

At Step 1120, the adjusting unit 170 sequentially acquires values of thevoltage Vopen while changing the magnitude of the adjustment currentIadjust generated by the adjustment current generating unit 1010, andacquires characteristics of the voltage Vopen (Vopen-Iadjustcharacteristics) generated corresponding to the adjustment currentIadjust.

At Step 1130, the adjusting unit 170 calculates a voltage Vopen_adjustfrom the characteristics of the voltage Vopen generated corresponding tothe adjustment current that are acquired at Step 1120. A method ofcalculating the voltage Vopen_adjust is described below.

At Step 1140, the adjusting unit 170 adjusts the reference voltageoutput by the reference voltage generating unit 120 based on thecharacteristics of the voltage Vopen which is the difference between thereference voltage and the output voltage of the sensor unit 110generated corresponding to the adjustment current, and ends the process.For example, the adjusting unit 170 adjusts the reference voltage outputby the reference voltage generating unit 120 such that the voltage Vopenbecomes the voltage Vopen_adjust calculated at Step 1130. Alternatively,the adjusting unit 170 may adjust the reference voltage output by thereference voltage generating unit 120 such that the voltage Vopen_adjustbecomes 0.

FIG. 12 illustrates characteristics of voltage Vopen generatedcorresponding to an input magnetic field Bin before and after magneticoperating point adjustment performed by the magnetic field measuringdevice 10 illustrated in FIG. 10 based on the flow illustrated in FIG.11. Since explanations similar to those related to FIG. 9 apply toportions given the same symbols as those illustrated in FIG. 9, thoseexplanations are omitted. A difference from FIG. 9 is that there is anapplied input magnetic field Bin which has a finite value (defined as“Bsignal”).

The switching unit 710 switches the state of control from theclosed-loop control to the open loop while the magnetic field generatingunit 140 is generating the feedback magnetic fields Bfeedback_initialand Bfeedback based on the feedback current. Then, since the feedbackmagnetic fields Bfeedback_initial and Bfeedback are no longer generated,the magnetic operating point of the first magnetoresistive element 112and the second magnetoresistive element 114 transitions to a point 1210based on the magnetic field Bsignal. In this state, the adjusting unit170 performs magnetic operating point adjustment based on the flowillustrated in FIG. 11 to adjust the reference voltage output by thereference voltage generating unit 120. A curve 1220 illustratescharacteristics of voltage Vopen generated corresponding to an inputmagnetic field Bin to the sensor unit 110 after magnetic operating pointadjustment performed by the magnetic field measuring device 10illustrated in FIG. 10 based on the flow illustrated in FIG. 11. Themagnetic field measuring device 10 illustrated in FIG. 10 performsmagnetic operating point adjustment based on the flow illustrated inFIG. 11 to thereby be able to cause a transition of the magneticoperating point of the first magnetoresistive element 112 and the secondmagnetoresistive element 114 from the point 1210 to a point 1230. Thispoint 1230 is a point where the voltage Vopen=Vopen_adjust, and if themagnetic field measuring device 10 illustrated in FIG. 10 switches thestate of control from the open loop to the closed loop in this state,and operates the first magnetoresistive element 112 and the secondmagnetoresistive element 114, the highest magnetic sensitivity can beattained.

FIG. 13 illustrates characteristics of voltage Vopen generatedcorresponding to an adjustment current Iadjust used by the adjustingunit 170 in the magnetic field measuring device 10 illustrated in FIG.10 for calculating voltage Vopen_adjust. The adjusting unit 170 acquiresvoltage Vopen characteristics generated corresponding to the adjustmentcurrent Iadjust like those illustrated by a curve 1320, for example,through Step 1120 illustrated in FIG. 11. Then, the adjusting unit 170calculates the voltage Vopen_adjust based on the curve 1320. Forexample, the adjusting unit 170 acquires, from the curve 1320, a voltageVopen_max which is the maximum of the voltage Vopen, and a voltageVopen_min which is the minimum of the voltage Vopen, and calculates theaverage value of the voltage Vopen_max and Vopen_min as the voltageVopen_adjust.

FIG. 14 illustrates characteristics of dVopen/dIadjust used by theadjusting unit 170 in the magnetic field measuring device 10 illustratedin FIG. 10 for calculating the voltage Vopen_adjust. The adjusting unit170 differentiates the curve 1320 with respect to the adjustment currentIadjust to thereby obtain characteristics of dVopen/dIadjust in responseto the adjustment current Iadjust like those illustrated in the curve1410, for example. Then, the adjusting unit calculates the voltage Vopenat a point 1420 where dVopen/dIadjust becomes the maximum as the voltageVopen_adjust.

FIG. 15 illustrates a flow for the magnetic field measuring device 10according to the present embodiment to measure a magnetic field. At Step1510, the magnetic field measuring device 10 magnetically resets thefirst magnetoresistive element 112 and the second magnetoresistiveelement 114. For example, before measurement of a measurement-targetmagnetic field by the magnetic field measuring unit 160, the adjustingunit 170 makes the reference voltage generating unit 120 generate areference voltage that makes the feedback current generating unit 130generate a reset magnetic field to magnetically saturate the firstmagnetoresistive element 112 and the second magnetoresistive element114, and thereafter the adjusting unit 170 adjusts the reference voltageback to its original value, and stops generation of the reset magneticfield by the magnetic field generating unit 140. In this manner, thisseries of operation of applying a reset magnetic field to magneticallysaturate the first magnetoresistive element 112 and the secondmagnetoresistive element 114 to the first magnetoresistive element 112and the second magnetoresistive element 114, and thereafter removing themagnetic field is defined as magnetic resetting.

Typically, a magnetoresistive element may have various magnetizationdirections in a magnetic domain depending on the history of inputmagnetic forces, but the magnetization directions in the magnetic domaincan be aligned once by performing magnetic resetting. Because of this,every time the magnetic field measuring device 10 measures a magneticfield, it can perform measurement under the same condition about thedirection of the magnetization in a magnetic domain, and therebymeasurement errors can be reduced. The magnetic resetting operationmentioned above can be executed, for example, at a time when the powersource of the magnetic field measuring device 10 is turned on, at a timewhen an unintended magnetic field is detected by the magnetic fieldmeasuring device 10, at a time when a predetermined period has elapsedafter a previous instance of magnetic resetting, at a time when thenumber of times of magnetic field measurement has reached apredetermined number of times after a previous instance of magneticresetting, or at other times.

Next, at Step 1520, the adjusting unit 170 adjusts the magneticoperating point of the first magnetoresistive element 112 and the secondmagnetoresistive element 114 based on a flow illustrated in at least anyone of FIGS. 3, 5, 8 and 11.

Next, at Step 1530, the magnetic field measuring unit 160 measures ameasurement-target magnetic field. Then, at Step 1540, the magneticfield measuring device 10 judges whether or not the number of times ofmagnetic field measurement has reached a predetermined number of times n(n is an integer equal to or larger than 1). If a result of thejudgement indicates that the number of times of magnetic fieldmeasurement is smaller than the predetermined number of times n, themagnetic field measuring device 10 returns to the process at Step 1530,and continues the processes. On the other hand, if a result of thejudgement indicates that the number of times of magnetic fieldmeasurement has reached the predetermined number of times n, themagnetic field measuring device 10 proceeds to the process at Step 1550,and at Step 1550, the magnetic field measuring unit 160 integratesmeasurement values obtained in a predetermined period, e.g., integratesn measurement values or performs another process, outputs a result ofthe integration, and ends the process. According to the presentembodiment, the magnetic field measuring unit 160 can obtain moreprecise output by integrating n measurement values, and outputting aresult of the integration.

Note that although in the explanation above, the magnetic fieldmeasuring device 10 returns to the process at Step 1530 if the number oftimes of measurement is smaller than the predetermined number of times nat Step 1540, instead of this, it may return to the process at Step 1520as illustrated by a dotted line in FIG. 15. That is, the magnetic fieldmeasuring device 10 may make the adjusting unit 170 adjust the magneticoperating point of the first magnetoresistive element 112 and the secondmagnetoresistive element 114 every time the magnetic field measuringunit 160 performs magnetic field measurement. By making the adjustingunit 170 adjust the magnetic operating point every time, it becomespossible to cause the first magnetoresistive element 112 and the secondmagnetoresistive element 114 to operate at a magnetic operating pointwhere it can achieve a higher magnetic sensitivity.

FIG. 16 illustrates the configuration of the magnetic field measuringdevice 10 provided with the switching unit 710 and a reset currentgenerating unit 1610 according to a variant of the magnetic fieldmeasuring device 10 of the present embodiment. The magnetic fieldmeasuring device 10 illustrated in this figure further includes thereset current generating unit 1610 in addition to the configurations ofthe magnetic field measuring device 10 illustrated in FIG. 7. Beforemeasurement of a measurement-target magnetic field performed by themagnetic field measuring unit 160, the reset current generating unit1610 supplies the magnetic field generating unit 140 with a resetcurrent that generates a reset magnetic field to magnetically saturatethe first magnetoresistive element 112 and the second magnetoresistiveelement 114. Although Step 1510 in the flow illustrated in FIG. 15illustrated an example in which the adjusting unit 170 magneticallyresets the first magnetoresistive element 112 and the secondmagnetoresistive element 114, as illustrated in this figure, the resetcurrent generating unit 1610 may be further provided, and a resetmagnetic field may be generated based on a reset current supplied by thereset current generating unit 1610 to magnetically reset the firstmagnetoresistive element 112 and the second magnetoresistive element114. In addition, if the magnetic field measuring device 10 has theadjustment current generating unit 1010 as illustrated in FIG. 10, thefunction of the reset current generating unit 1610 may be realized bythe adjustment current generating unit 1010. In addition, if themagnetic field measuring device 10 has the variable resistor 224 in thereference voltage generating unit 120, a reset magnetic field can alsobe generated by changing the resistance value of the variable resistor224. For example, as in the embodiment illustrated in FIG. 2, since ifthe reference voltage generating unit 120 has the fixed resistor 222 andthe variable resistor 224, and generates a reset magnetic field byadjusting the resistance value of the variable resistor 224, it is notrequired to use the reset current generating unit 1610, the system canbe simplified. Here, in order to attain the reference voltage forgenerating a reset magnetic field, preferably, the output voltage rangeof the reference voltage generating unit 120 is larger than the outputvoltage range of the sensor unit 110. Note that the output voltage rangeis defined as the difference between the maximum value that the outputvoltage can assume and the minimum value that the output voltage canassume.

FIG. 17 is a figure illustrating how voltage Vclosed changes when theresistance value of the variable resistor 224 is changed in theembodiment illustrated in FIG. 2. The voltage Vclosed shows smallchanges if the variable resistance value is between Rsat,lo and Rsat,up.But if the variable resistance value is equal to or smaller than Rsat,loor is equal to or larger than Rsat,up, the voltage Vclosed becomes verylarge or small, and for example the output value of the magnetic fieldmeasuring unit 160 (AD value of the voltage Vclosed) overflows (stays atthe upper limit or lower limit of the output), or almost overflows. Thatis, the closed loop of the magnetic field measuring device 10 operatesnormally if the variable resistance value is between Rsat,lo andRsat,up. But since the magnetoresistive elements are magneticallysaturated, and an input magnetic field to the sensor unit 110 cannot becancelled out normally by a feedback magnetic field if the variableresistance value is equal to or smaller than Rsat,lo or is equal to orlarger than Rsat,up, the magnetic field measuring device 10 does notoperate in the closed loop. Accordingly, magnetic resetting of themagnetoresistive elements can be performed by making the variableresistance value equal to or smaller than Rsat,lo or equal to or largerthan Rsat,up. Here, Rsat,up is defined as the upper reset magnetic fieldgenerating resistance value, and the output voltage of the referencevoltage generating unit 120 generated at this time is defined as theupper reset magnetic field generating voltage. In addition, Rsat,lo isdefined as the lower reset magnetic field generating resistance value,and the output voltage of the reference voltage generating unit 120generated at this time is defined as the lower reset magnetic fieldgenerating voltage.

The output range (the closed loop operation output range) of the voltageVclosed in which it operates in the closed loop may be set asVclosed,sat_up to Vclosed,sat_lo, and the magnetic field measuringdevice 10 may measure the voltage Vclosed while changing the resistanceof the variable resistor 224 in the reference voltage generating unit120, and may detect resistance values that generated output exceedingthis output range as Rsat,up and Rsat,lo, respectively. Then, theadjusting unit 170 may adjust the magnetic operating point of themagnetoresistive elements based on the upper reset magnetic fieldgenerating resistance value Rsat,up or the lower reset magnetic fieldgenerating resistance value Rsat,lo. For example, the adjusting unit 170may adjust the variable resistor 224 such that the magnetoresistiveelements operate at a magnetic operating point at which it cantheoretically attain a high magnetic resolution. Then, the resistance ofsuch a magnetoresistive element (one element) is the resistance valuewhich is shifted by about ½ to ¼ of the range of resistance changes thatare observed as a response to a magnetic field from the high resistanceside of the range. In view of this, the resistance value of the variableresistor 224 may be set to a resistance value of ½ to ¼ (e.g., ¼ orlarger and ½ or smaller) of the range of the upper reset magnetic fieldgenerating resistance value Rsat,up and the lower reset magnetic fieldgenerating resistance value Rsat,lo that generate reset magnetic fieldgenerating voltages. For example, since resistance change curves thatare symmetric with respect to a magnetic field overlap, if there are twomagnetoresistive elements, the resistance value of the variable resistor224 may be set to the mean value (½) of the upper reset magnetic fieldgenerating resistance value Rsat,up and the lower reset magnetic fieldgenerating resistance value Rsat,lo. In addition, for example, if thereis one magnetoresistive element, the resistance value of the variableresistor 224 may be set to a resistance value of ½ to ¼ (e.g., ¼ orlarger and ½ or smaller) of the range of the upper reset magnetic fieldgenerating resistance value Rsat,up and the lower reset magnetic fieldgenerating resistance value Rsat,lo. Note that this resistance value isobtained if the variable resistor 224 is at an arrangement positionwhere one of the terminals of the variable resistor 224 is on the sideof the fixed resistor 222 and the other terminal is on the GND side asillustrated in FIG. 2, and if the latter terminal is arranged on the VCCside, the resistance value may be a resistance value of ½ to ¾ of therange of the upper reset magnetic field generating resistance valueRsat,up and the lower reset magnetic field generating resistance valueRsat,lo. Since, in this manner, the magnetic field measuring device 10can adjust the magnetic operating point of the magnetoresistive elementsby changing the variable resistor 224, it can make the magnetoresistiveelements operate at a point where a high magnetic resolution can beattained. Stated differently, before measurement of a measurement-targetmagnetic field performed by the magnetic field measuring unit 160, theadjusting unit 170: changes the resistance value of the variableresistor 224 provided in the reference voltage generating unit 120;makes the reference voltage generating unit 120 generate reset magneticfield generating voltages (upper reset magnetic field generating voltageand lower reset magnetic field generating voltage) that generate thereset magnetic field; and adjusts the reference voltage based on theresistance values of the variable resistor 224 that generate the resetmagnetic field generating voltages. That is, the magnetic fieldmeasuring device 10 changes the resistance value of the variableresistor 224 while measuring the voltage Vclosed so as to detect theupper reset magnetic field generating resistance value Rsat,up and thelower reset magnetic field generating resistance value Rsat,lo, andadjusts the voltage generated by the reference voltage generating unit120 based on the detected upper reset magnetic field generatingresistance value Rsat,up and the lower reset magnetic field generatingresistance value Rsat,lo. At this time, the magnetic field measuringdevice 10 may first change the resistance value of the variable resistor224 so as to detect the upper reset magnetic field generating resistancevalue Rsat,up, or may first change the resistance value of the variableresistor 224 so as to detect the lower reset magnetic field generatingresistance value Rsat,lo. In addition, if the MR ratio (MRratio=Rsat,up/Rsat,lo) of a magnetoresistive element is known inadvance, the magnetic field measuring device 10 may adjust the voltagegenerated by the reference voltage generating unit 120 based on theknown MR ratio after detecting any one of the upper reset magnetic fieldgenerating resistance value Rsat,up and the lower reset magnetic fieldgenerating resistance value Rsat,lo. Note that, by changing thearrangement position of the variable resistor 224 as explained above,the resistance value of the variable resistor 224 is set to a resistancevalue of either ½ to ¾ or ½ to ¼ of the range of the upper resetmagnetic field generating resistance value Rsat,up and the lower resetmagnetic field generating resistance value Rsat,lo. Such cases are alsoaspects of the present embodiment.

FIG. 18 illustrates the configuration of the magnetic field measuringdevice 10 provided with a switch 1810 and a high-pass filter 1820according to a variant of the magnetic field measuring device 10 of thepresent embodiment. The magnetic field measuring device 10 illustratedin this figure includes the switch 1810 and high-pass filter 1820 inaddition to the configurations of the magnetic field measuring device 10illustrated in FIG. 1. The switch 1810 is provided between the secondoperational amplifier 154 and the AD converter 156, and switches whetherto supply an output voltage VAMP of the second operational amplifier 154directly to the AD converter 156 or to supply output of the secondoperational amplifier to the AD converter 156 via the high-pass filter1820. The high-pass filter 1820 allows passage therethrough ofhigh-frequency components of the output voltage VAMP of the secondoperational amplifier 154, and supplies them to the AD converter 156.

The magnetic field measuring device 10 illustrated in this figureswitches the switch 1810 to supply the output voltage VAMP of the secondoperational amplifier 154 directly to the AD converter 156 bypassing thehigh-pass filter 1820 in an adjustment phase, and supplies the outputvoltage VAMP of the second operational amplifier 154 to the AD converter156 via the high-pass filter 1820 in a measurement phase. Thereby, if ameasurement-target magnetic field is AC components in a measurementphase, unnecessary DC components can be blocked, and the magnetic fieldmeasuring unit 160 can measure the measurement-target magnetic fieldmore precisely.

FIG. 19 illustrates the configuration of the magnetic field measuringdevice 10 provided with a third operational amplifier 1910 according toa variant of the magnetic field measuring device 10 of the presentembodiment. The magnetic field measuring device 10 illustrated in thisfigure further includes the third operational amplifier 1910 in additionto the configurations of the magnetic field measuring device 10illustrated in FIG. 1, and the feedback current generating unit 130 isformed by using two or more operational amplifiers. The thirdoperational amplifier 1910 has one differential input terminal connectedto output of the first operational amplifier 132, and anotherdifferential input terminal connected to the fixed voltage source 1920.The feedback current generating unit 130 illustrated in this figure maymake the first operational amplifier 132 output the voltage Vopen whichis the difference between the reference voltage and the output voltageof the sensor unit 110, and make the third operational amplifier 1910generate a feedback current based on the difference between the voltageVopen and the fixed voltage 2. Here, the same voltage may be set for thefixed voltage 1 and the fixed voltage 2.

FIG. 20 illustrates a specific example of a sensor unit 110 according tothe present embodiment. The sensor unit 110 has a magnetoresistiveelement 2010, and magnetic flux concentrators 2020 and 2030 (themagnetic flux concentrator 2020 and the magnetic flux concentrator 2030are collectively referred to as a “magnetic flux concentrating unit”)that are arranged at both ends of the magnetoresistive element 2010.Note that, here, the magnetoresistive element 2010 may be at least oneof the first magnetoresistive element 112 and the secondmagnetoresistive element 114, for example. The magnetic fluxconcentrators 2020 and 2030 are arranged at both ends of themagnetoresistive element 2010 so as to sandwich the magnetoresistiveelement 2010. That is, the sensor unit 110 includes the magnetic fluxconcentrating unit arranged adjacent to the magnetoresistive element2010. In this figure, the magnetic flux concentrator 2020 is provided onthe negative side of the magnetoresistive element 2010 along themagnetosensitive axis, and the magnetic flux concentrator 2030 isprovided on the positive side of the magnetoresistive element 2010 alongthe magnetosensitive axis. Note that, here, the magnetosensitive axismay lie along the direction of magnetization that is fixed by amagnetization fixed layer forming the magnetoresistive element 2010. Inaddition, if a magnetic field is input from the negative side of themagnetosensitive axis toward its positive side, the resistance of themagnetoresistive element 2010 may increase or decrease. The magneticflux concentrators 2020 and 2030 are formed of a material having highmagnetic permeability such as Permalloy, for example. Then, if thesensor unit 110 is configured in the manner as illustrated in thepresent specific example, the coil 142 is wound to surroundcross-sections of the magnetoresistive element 2010, and the magneticflux concentrators 2020 and 2030 arranged at both ends of themagnetoresistive element 2010. That is, the feedback current generatingunit 130 is formed to surround the magnetoresistive element 2010 and themagnetic flux concentrating unit. In addition, if the sensor unit 110has a plurality of magnetoresistive elements 2010, it may have aplurality of sets of a magnetoresistive element and magnetic fluxconcentrators arranged at both ends thereof In this case, one coil 142may be wound to surround a set of a magnetoresistive element andmagnetic flux concentrators arranged at both ends thereof.

If such a sensor unit 110 receives a magnetic field from the negativeside of the magnetosensitive axis to its positive side, the magneticflux concentrators 2020 and 2030 formed of a material having highmagnetic permeability are magnetized to thereby generate a magnetic fluxdistribution like the one indicated by broken lines in this figure.Then, magnetic fluxes generated by magnetization of the magnetic fluxconcentrators 2020 and 2030 pass through the position of themagnetoresistive element 2010 sandwiched between the two magnetic fluxconcentrators 2020 and 2030. Because of this, the magnetic flux densityat the position of the magnetoresistive element 2010 can besignificantly increased by arranging the magnetic flux concentrators2020 and 2030. In addition, as in the present specific example, by usingthe magnetoresistive element 2010 arranged at a position with a smallarea sandwiched by the magnetic flux concentrators 2020 and 2030 toperform sampling of the spatial distribution of a magnetic field, itbecomes possible to make a sampling point in the space clear.

FIG. 21 illustrates a magnetic flux distribution observed when afeedback magnetic field is generated to the sensor unit 110 according tothe present specific example. In FIG. 21, members having the samefunctions and configurations as those of members illustrated in FIG. 20are given the same symbols, and also explanations related to mattersother than differences therebetween are omitted hereinafter. If afeedback current is supplied to the coil 142 in the sensor unit 110according to the present specific example, the coil 142 generates afeedback magnetic field to thereby generate a magnetic flux distributionlike the one illustrated by alternate long and short dash lines in thisfigure. Magnetic fluxes generated by this feedback magnetic field arespatially distributed to cancel out the spatial distribution of amagnetic field input to the magnetoresistive element 2010 andmagnetically amplified by the magnetic flux concentrators 2020 and 2030.Because of this, as illustrated in the present specific example, if themagnetic flux concentrators 2020 and 2030 are arranged at both ends ofthe magnetoresistive element 2010, the sensor unit 110 can accuratelycancel out the magnetic field distribution at the position of themagnetoresistive element 2010 by using the feedback magnetic field; as aresult, it becomes possible to realize a sensor with high linearitybetween an input magnetic field and an output voltage.

FIG. 22 illustrates an exemplary configuration of the sensor unit 110according to the present specific example. In FIG. 22, members havingthe same functions and configurations as those of members illustrated inFIG. 20 are given the same symbols, and also explanations related tomatters other than differences therebetween are omitted hereinafter. Inthis figure, the magnetoresistive element 2010 has a magnetization freelayer 2110 and a magnetization fixed layer 2120. Typically, themagnetoresistive element 2010 has a structure in which two ferromagneticlayers sandwich an insulator thin-film layer. The magnetization freelayer 2110 is a layer which is one of the two ferromagnetic layers, andhas a magnetization direction that changes depending on an externalmagnetic field. In addition, the magnetization fixed layer 2120 is alayer which is the other of the two ferromagnetic layers, and has amagnetization direction which does not change even if it receives anexternal magnetic field. For example, the magnetoresistive element 2010has the magnetization free layer 2110, a non-magnetic layer, and themagnetization fixed layer 2120 that are stacked on a substrate in thisorder.

In the present specific example, the magnetoresistive element 2010 is amagnetoresistive element having a so-called bottom free structure inwhich the magnetization free layer 2110 is arranged at a lower portion,and the magnetization fixed layer 2120 is arranged at an upper portionof the magnetization free layer 2110 via an insulator thin-film layer(not illustrated). Since a magnetoresistive element with the bottom freestructure allows the magnetization free layer 2110 to be formed to havea relatively wide area, a high magnetic sensitivity can be attained.Note that, in the magnetoresistive element 2010, when seen from above,the area of the magnetization fixed layer 2120 is smaller than the areaof the magnetization free layer 2110, and the magnetosensitive area isdetermined based on the area of the magnetization fixed layer 2120.

In addition, in the present specific example, the sensor unit 110 hasthe magnetic flux concentrators 2020 and 2030 that are arranged at bothends of the magnetoresistive element 2010 so as to sandwich themagnetoresistive element 2010 at the middle of their interval, via aninsulation layer (not illustrated) at an upper portion of themagnetoresistive element 2010. Thereby, the magnetoresistive element2010 is arranged in a small space sandwiched by the magnetic fluxconcentrators 2020 and 2030.

Here, in this figure, the length of the magnetization free layer 2110along the magnetosensitive axis direction is defined as a magnetizationfree layer length L_Free. In addition, the length of the magnetizationfree layer 2110 along an axis perpendicular to the magnetosensitive axisdirection when seen from above is defined as a magnetization free layerwidth W_Free. In addition, the length of the magnetization fixed layer2120 along the magnetosensitive axis direction is defined as amagnetization fixed layer length L_Pin. In addition, the length of themagnetization fixed layer 2120 along an axis perpendicular to themagnetosensitive axis direction when seen from above is defined as amagnetization fixed layer width W_Pin. In addition, the length from oneouter end of a magnetic flux concentrator to one outer end of themagnetization free layer along the magnetosensitive axis direction (inthis figure, the length from the left end of the magnetic fluxconcentrator 2020 to its right end along the magnetosensitive axisdirection, and the length from the right end of the magnetic fluxconcentrator 2030 to its left end along the magnetosensitive axisdirection) is defined as a magnetic flux concentrator length L_FC. Inaddition, the length of the magnetic flux concentrator along an axisperpendicular to the magnetosensitive axis direction when seen fromabove is defined as a magnetic flux concentrator width W_FC. Inaddition, the length of the magnetic flux concentrator along an axisperpendicular to the magnetosensitive axis direction when seen from sideis defined as a magnetic flux concentrator thickness T_FC. In addition,the interval between the two magnetic flux concentrators 2020 and 2030along the magnetosensitive axis direction (in this figure, the lengthfrom the right end of the magnetic flux concentrator 2020 to the leftend of the magnetic flux concentrator 2030 along the magnetosensitiveaxis direction) is defined as a magnetic flux concentrator intervalG_FC. In addition, an interval from the center of the magnetization freelayer 2110 in its thickness direction to the bottom surface of themagnetic flux concentrator along an axis perpendicular to themagnetosensitive axis direction when seen from side is defined as amagnetic flux concentrator height H_FC.

Various embodiments of the present invention may be described withreference to flowcharts and block diagrams whose blocks may represent(1) steps of processes in which operations are performed or (2) sectionsof apparatuses responsible for performing operations. Certain steps andsections may be implemented by dedicated circuitry, programmablecircuitry supplied with computer-readable instructions stored oncomputer-readable media, and/or processors supplied withcomputer-readable instructions stored on computer-readable media.Dedicated circuitry may include digital and/or analog hardware circuitsand may include integrated circuits (IC) and/or discrete circuits.Programmable circuitry may include reconfigurable hardware circuitscomprising logical AND, OR, XOR, NAND, NOR, and other logicaloperations, flip-flops, registers, memory elements, etc., such asfield-programmable gate arrays (FPGA), programmable logic arrays (PLA),etc.

Computer-readable media may include any tangible device that can storeinstructions for execution by a suitable device, such that thecomputer-readable medium having instructions stored therein comprises anarticle of manufacture including instructions which can be executed tocreate means for performing operations specified in the flowcharts orblock diagrams. Examples of computer-readable media may include anelectronic storage medium, a magnetic storage medium, an optical storagemedium, an electromagnetic storage medium, a semiconductor storagemedium, etc. More specific examples of computer-readable media mayinclude a floppy disk, a diskette, a hard disk, a random access memory(RAM), a read-only memory (ROM), an erasable programmable read-onlymemory (EPROM or Flash memory), an electrically erasable programmableread-only memory (EEPROM), a static random access memory (SRAM), acompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a BLU-RAY® disc, a memory stick, an integrated circuit card, etc.

Computer-readable instructions may include assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, JAVA (registeredtrademark), C++, etc., and conventional procedural programminglanguages, such as the “C” programming language or similar programminglanguages.

Computer-readable instructions may be provided to a processor of ageneral purpose computer, special purpose computer, or otherprogrammable data processing apparatus, or to programmable circuitry,locally or via a local area network (LAN), wide area network (WAN) suchas the Internet, etc., to execute the computer-readable instructions tocreate means for performing operations specified in the flowcharts orblock diagrams. Examples of processors include computer processors,processing units, microprocessors, digital signal processors,controllers, microcontrollers, etc.

FIG. 23 shows an example of a computer 2200 in which aspects of thepresent invention may be wholly or partly embodied. A program that isinstalled in the computer 2200 can cause the computer 2200 to functionas or perform operations associated with apparatuses of the embodimentsof the present invention or one or more sections thereof, and/or causethe computer 2200 to perform processes of the embodiments of the presentinvention or steps thereof. Such a program may be executed by the CPU2212 to cause the computer 2200 to perform certain operations associatedwith some or all of the blocks of flowcharts and block diagramsdescribed herein.

The computer 2200 according to the present embodiment includes a CPU2212, a RAM 2214, a graphics controller 2216, and a display device 2218,which are mutually connected by a host controller 2210. The computer2200 also includes input/output units such as a communication interface2222, a hard disk drive 2224, a DVD-ROM drive 2226 and an IC card drive,which are connected to the host controller 2210 via an input/outputcontroller 2220. The computer also includes legacy input/output unitssuch as a ROM 2230 and a keyboard 2242, which are connected to theinput/output controller 2220 through an input/output chip 2240.

The CPU 2212 operates according to programs stored in the ROM 2230 andthe RAM 2214, thereby controlling each unit. The graphics controller2216 obtains image data generated by the CPU 2212 on a frame buffer orthe like provided in the RAM 2214 or in itself, and causes the imagedata to be displayed on the display device 2218.

The communication interface 2222 communicates with other electronicdevices via a network. The hard disk drive 2224 stores programs and dataused by the CPU 2212 within the computer 2200. The DVD-ROM drive 2226reads the programs or the data from the DVD-ROM 2201, and provides thehard disk drive 2224 with the programs or the data via the RAM 2214. TheIC card drive reads programs and data from an IC card, and/or writesprograms and data into the IC card.

The ROM 2230 stores therein a boot program or the like executed by thecomputer 2200 at the time of activation, and/or a program depending onthe hardware of the computer 2200. The input/output chip 2240 may alsoconnect various input/output units via a parallel port, a serial port, akeyboard port, a mouse port, and the like to the input/output controller2220.

A program is provided by computer readable media such as the DVD-ROM2201 or the IC card. The program is read from the computer readablemedia, installed into the hard disk drive 2224, RAM 2214, or ROM 2230,which are also examples of computer readable media, and executed by theCPU 2212. The information processing described in these programs is readinto the computer 2200, resulting in cooperation between a program andthe above-mentioned various types of hardware resources. An apparatus ormethod may be constituted by realizing the operation or processing ofinformation in accordance with the usage of the computer 2200.

For example, when communication is performed between the computer 2200and an external device, the CPU 2212 may execute a communication programloaded onto the RAM 2214 to instruct communication processing to thecommunication interface 2222, based on the processing described in thecommunication program. The communication interface 2222, under controlof the CPU 2212, reads transmission data stored on a transmissionbuffering region provided in a recording medium such as the RAM 2214,the hard disk drive 2224, the DVD-ROM 2201, or the IC card, andtransmits the read transmission data to a network or writes receptiondata received from a network to a reception buffering region or the likeprovided on the recording medium.

In addition, the CPU 2212 may cause all or a necessary portion of a fileor a database to be read into the RAM 2214, the file or the databasehaving been stored in an external recording medium such as the hard diskdrive 2224, the DVD-ROM drive 2226 (DVD-ROM 2201), the IC card, etc.,

-   The CPU 2212 may then write back the processed data to the external    recording medium.

Various types of information, such as various types of programs, data,tables, and databases, may be stored in the recording medium to undergoinformation processing. The CPU 2212 may perform various types ofprocessing on the data read from the RAM 2214, which includes varioustypes of operations, processing of information, condition judging,conditional branch, unconditional branch, search/replace of information,etc., as described throughout this disclosure and designated by aninstruction sequence of programs, and writes the result back to the RAM2214. In addition, the CPU 2212 may search for information in a file, adatabase, etc., in the recording medium. For example, when a pluralityof entries, each having an attribute value of a first attributeassociated with an attribute value of a second attribute, are stored inthe recording medium, the CPU 2212 may search for an entry matching thecondition whose attribute value of the first attribute is designated,from among the plurality of entries, and read the attribute value of thesecond attribute stored in the entry, thereby obtaining the attributevalue of the second attribute associated with the first attributesatisfying the predetermined condition.

The above-explained program or software modules may be stored in thecomputer readable media on or near the computer 2200. In addition, arecording medium such as a hard disk or a RAM provided in a serversystem connected to a dedicated communication network or the Internetcan be used as the computer readable media, thereby providing theprogram to the computer 2200 via the network.

While the embodiments of the present invention have been described, thetechnical scope of the invention is not limited to the above describedembodiments. It is apparent to persons skilled in the art that variousalterations and improvements can be added to the above-describedembodiments. It is also apparent from the scope of the claims that theembodiments added with such alterations or improvements can be includedin the technical scope of the invention.

The operations, procedures, steps, and stages of each process performedby an apparatus, system, program, and method shown in the claims,embodiments, or diagrams can be performed in any order as long as theorder is not indicated by “prior to,” “before,” or the like and as longas the output from a previous process is not used in a later process.Even if the process flow is described using phrases such as “first” or“next” in the claims, embodiments, or diagrams, it does not necessarilymean that the process must be performed in this order.

What is claimed is:
 1. A magnetic field measuring device comprising: asensor unit that has at least one magnetoresistive element; a referencevoltage generating unit that outputs a reference voltage; a magneticfield generating unit that generates a magnetic field to be applied tothe sensor unit; a feedback current generating unit that supplies,according to a difference between an output voltage of the sensor unitand the reference voltage, the magnetic field generating unit with afeedback current that generates a feedback magnetic field to diminish aninput magnetic field to the sensor unit; a magnetic field measuring unitthat outputs a measurement value corresponding to the feedback current;and an adjusting unit that uses the output voltage of the sensor unit toadjust the reference voltage.
 2. The magnetic field measuring deviceaccording to claim 1, wherein in an adjustment phase, the adjusting unitadjusts the reference voltage; and in a measurement phase, the magneticfield measuring unit outputs a measurement value corresponding to thefeedback current generated for a measurement-target magnetic field. 3.The magnetic field measuring device according to claim 1, wherein thereference voltage generating unit has at least one variable resistor,and the adjusting unit changes a resistance value of the variableresistor to adjust the reference voltage.
 4. The magnetic fieldmeasuring device according to claim 1, wherein the adjusting unitadjusts the reference voltage based on the feedback current.
 5. Themagnetic field measuring device according to claim 4, wherein, upon thesensor unit receiving an adjustment magnetic field, the adjusting unitadjusts the reference voltage such that the measurement value fallswithin a predetermined range as a result of the reception of theadjustment magnetic field.
 6. The magnetic field measuring deviceaccording to claim 4, wherein the adjusting unit adjusts the referencevoltage so as to lower a variance of the feedback current.
 7. Themagnetic field measuring device according to claim 1, further comprisinga switching unit that switches whether to or not to supply the feedbackcurrent to the magnetic field generating unit, wherein the adjustingunit uses the output voltage of the sensor unit generated while thefeedback current is not being supplied to the magnetic field generatingunit to adjust the reference voltage.
 8. The magnetic field measuringdevice according to claim 7, wherein, upon the sensor unit receiving anadjustment magnetic field while the feedback current is not beingsupplied to the magnetic field generating unit, the adjusting unitadjusts the reference voltage such that the difference between theoutput voltage of the sensor unit and the reference voltage falls withina predetermined range as a result of the reception of the adjustmentmagnetic field.
 9. The magnetic field measuring device according toclaim 7, further comprising an adjustment current generating unit thatgenerates an adjustment current, wherein the switching unit supplies theadjustment current to the magnetic field generating unit if the feedbackcurrent is not supplied to the magnetic field generating unit, and theadjusting unit uses the output voltage of the sensor unit generatedwhile the adjustment current is being supplied to the magnetic fieldgenerating unit to adjust the reference voltage.
 10. The magnetic fieldmeasuring device according to claim 9, wherein the adjusting unitadjusts the reference voltage based on a characteristic of thedifference between the reference voltage and the output voltage of thesensor unit generated corresponding to the adjustment current.
 11. Themagnetic field measuring device according to claim 1, wherein themagnetic field measuring unit integrates measurement values obtained ina predetermined period, and outputs the integrated measurement values.12. The magnetic field measuring device according to claim 1, wherein,before measurement of a measurement-target magnetic field performed bythe magnetic field measuring unit, the adjusting unit makes thereference voltage generating unit generate the reference voltage thatmakes the feedback current generating unit generate a reset magneticfield to magnetically saturate the magnetoresistive element.
 13. Themagnetic field measuring device according to claim 12, wherein beforemeasurement of a measurement-target magnetic field performed by themagnetic field measuring unit, the adjusting unit: changes a resistancevalue of a variable resistor provided in the reference voltagegenerating unit to make the reference voltage generating unit generate areset magnetic field generating voltage that generates the resetmagnetic field; and adjusts the reference voltage based on theresistance value of the variable resistor that generates the resetmagnetic field generating voltage.
 14. The magnetic field measuringdevice according to claim 13, wherein the adjusting unit adjusts thereference voltage by using the resistance value of the variable resistorwhich is set to ½ to ¼ of a range of an upper reset magnetic fieldgenerating resistance value and a lower magnetic field generatingresistance value each of which generates the reset magnetic fieldgenerating voltage.
 15. The magnetic field measuring device according toclaim 12, wherein an output voltage range of the reference voltagegenerating unit is larger than an output voltage range of the sensorunit.
 16. The magnetic field measuring device according to claim 1,further comprising a reset current generating unit that supplies, beforemeasurement of a measurement-target magnetic field performed by themagnetic field measuring unit, the magnetic field generating unit with areset current that generates a reset magnetic field to magneticallysaturate the magnetoresistive element.
 17. The magnetic field measuringdevice according to claim 1, further comprising a high-pass filter thatallows passage therethrough of a high-frequency component of ameasurement value output by the magnetic field measuring unit.
 18. Themagnetic field measuring device according to claim 1, wherein thefeedback current generating unit is formed by using two or moreoperational amplifiers.
 19. The magnetic field measuring deviceaccording to claim 1, wherein the sensor unit includes a magnetic fluxconcentrating unit arranged adjacent to the magnetoresistive element,and the feedback current generating unit is formed to surround themagnetoresistive element and the magnetic flux concentrating unit. 20.The magnetic field measuring device according to claim 1, wherein themagnetoresistive element includes a magnetization free layer, anon-magnetic layer, and a magnetization fixed layer that are stacked ona substrate in this order, and, when seen from above, the area of themagnetization fixed layer is smaller than the area of the magnetizationfree layer, and a magnetosensitive area is determined based on the areaof the magnetization fixed layer.
 21. The magnetic field measuringdevice according to claim 1, wherein the sensor unit has a firstmagnetoresistive element and a second magnetoresistive element that areconnected in series and have opposite polarity to each other, and avoltage across the first magnetoresistive element and the secondmagnetoresistive element is output.
 22. A magnetic field measurementmethod by which a magnetic field measuring device measures a magneticfield, the magnetic field measurement method comprising: supplying, bythe magnetic field measuring device and according to a differencebetween an output voltage of a sensor unit having at least onemagnetoresistive element and a reference voltage output by a referencevoltage generating unit, a magnetic field generating unit that generatesa magnetic field to be applied to the sensor unit with a feedbackcurrent that generates a feedback magnetic field to diminish an inputmagnetic field to the sensor unit; outputting a measurement valuecorresponding to the feedback current by the magnetic field measuringdevice; and adjusting, by the magnetic field measuring device, thereference voltage based on the output voltage of the sensor unit.
 23. Arecording medium having recorded thereon a magnetic field measurementprogram that, when executed by a computer, makes the computer functionas: a feedback current generating unit that supplies, according to adifference between an output voltage of a sensor unit having at leastone magnetoresistive element and a reference voltage output by areference voltage generating unit, a magnetic field generating unit thatgenerates a magnetic field to be applied to the sensor unit with afeedback current that generates a feedback magnetic field to diminish aninput magnetic field to the sensor unit; a magnetic field measuring unitthat outputs a measurement value corresponding to the feedback current;and an adjusting unit that uses the output voltage of the sensor unit toadjust the reference voltage.